The main benefit that you will get from migrating to PHP from Perl cgi is the same benefit that you would get migrating to mod_perl: speed. I've noticed a certain painful shock that Perl programmers go through when they first have to use other languages such as Java or PHP. Usually it's accompanied by occasional exclamations such as "G___ d___ it, why the f____ is this so f___ing complicated. I could f___ing do this in one mother-f___ing line of Perl. Why the f___ do they have to write this piece of s___ in <INSERT LANAGUAGE HERE> anyway?" etc etc... In calmer moments around the watercooler they attempt to make up for their past public rudeness by extolling to everyone tales of the wonderful things that they did with Perl in the past.
PHP is a lot better compared to my days when they paid me to program in PHP2. It's does, however, still have a long way to go to catch up with the CPAN.